Bony balancing apparatus and method for total knee replacement

ABSTRACT

Total knee replacement surgery is improved through custom cuts on the distal femur without resorting to expensive computer navigation. The method involves measurement, on plain radiographs prior to surgery, the amount of bone that would be resected on each knee, medially and laterally on the distal femur. In the preferred embodiments, the predetermined distance is in the range of 8-10 mm, more preferably 10 mm, and the resulting distance from the second line to the apex of the lateral condyle is in the range of 6 to 7 mm. A cutting fixture is provided and used to resect the medial condyle at the predetermined distance and the lateral condyle at the measured resulting distance.

FIELD OF THE INVENTION

This invention relates generally to total knee replacement and, in particular to surgical techniques and instrumentation to more precisely balance the knee joint.

BACKGROUND OF THE INVENTION

In classical arthroplasty alignment, the distal femoral cut is perpendicular to the femoral mechanical axis. This axis is defined as a line drawn through the center of the femoral head to the center of the knee, which is defined by the center of the intercondylar notch, with the patient standing and weight bearing. Under current practice, most surgeons prefer a zero degree cut; that is, a perpendicular cut on the tibia. This is a line from the center of the tibial anatomy, which is at the center of the tibial spines, to the center of the talus. The goal here is a right angle cut to that alignment axis. When these two landmarks are combined, the result should be a well-balanced knee in the coronal plane, meaning with the knee in full extension at 30 and 60 degrees of flexion. There should also be minimal to trace laxity in that knee on examination at the time of the arthroplasty procedure.

Historically, if the alignment is not appropriate and there is imbalance present after the bone cuts have been made, most surgeons assume that there are problems with the soft tissue balancing and that there are contractures of the medial or lateral structures which are preventing perfect balancing of the knee. While this may be true in a small portion of knees with significant combined sagittal and coronal deformity, I believe that improper bone cuts are the problem and not ligament balancing. I believe this is true in 95 percent of total knee replacement cases.

Currently, most surgeons use instrumentation which has been around for approximately 40 years. Using these instruments on the femoral side, surgeons first position an intramedullary rod, which is inserted through a hole drilled in the distal femur. After the rod is placed, a five degree valgus alignment for the cuts on the distal femur is arbitrarily and traditionally chosen for most male patients. This five degree valgus alignment is chosen because typically in men the difference between the anatomic axis, which is the line drawn up the femoral shaft to the center of the femur, and the mechanical axis, which is the line drawn from the center of the femoral head to the center of the femur is roughly five degrees. This difference in women it is approximately seven degrees. Thus, when using the intramedullary alignment on the femur, this angle is chosen for the cut on the distal femur to achieve proper alignment.

However, as learned from computer navigation, these historically elected angles may be inappropriate most of the time. Some patients may exhibit 3.8 degrees of valgus in their alignment, others may be 7.3. The fact is, true accuracy can never be realized with an educated guess. Computer navigation is perhaps the most accurate way to make this cut. Navigated procedures actually locate the center of the femoral head based on complex mathematic algorithms, and “registration points” on the bones taken at the time of surgery, using existing, identifiable landmarks. However, computer navigation is expensive, it is time consuming, and it will probably not ever be available to every surgeon.

SUMMARY OF THE INVENTION

This invention improves upon total knee replacement surgery by facilitating custom cuts on the distal femur without resorting to expensive computer navigation. The method involves measurement, on plain radiographs prior to surgery, the amount of bone that would be resected on each knee, medially and laterally on the distal femur, based upon the perpendicular measurement from the center of the femoral head to the center of the intercondylar notch. The anatomy of each patient is unique, and each requires a unique amount of resection. With the exception of computer navigation, all other existing approaches merely guess at these amounts.

The method begins with obtaining an x-ray image of a patient's femur, the image including a hip joint with a femoral head and a knee joint with medial and lateral condyles and an intercondylar notch. The image is preferably a digital, standing x-ray image. A first line is identified on the image from the center of the femoral head to the center of the distal femur and the intercondylar notch. A second line is traced perpendicular to the first line, the second line being at a predetermined, arbitrary distance from the apex of the medial condyle, preferably 8-10 mm. The resulting distance from the second line to the apex of the lateral condyles read off of the x-ray image. A cutting fixture is provided and used to resect the medial condyle at the predetermined distance and the lateral condyle at the measured resulting distance.

In the preferred embodiments, the predetermined distance is in the range of 8-10 mm, more preferably 10 mm, and the resulting distance from the second line to the apex of the lateral condyle is in the range of 6 to 7 mm—based upon clinical research, the amount removed from the lateral condyle is in the range of 3-4 mm less than the medial side.

The cutting fixture may be initially rotatable to first adjust for the predetermined distance, and/or may further include an arm configured for placement on the outer cortex of a femur. The cutting fixture includes at least one cutting slot and one or more devices for measuring the distance between the apex of the medial condyle and the slot for resecting the medial condyle and for measuring the distance between the apex of the lateral condyle and the slot for resecting the lateral condyle. In some embodiments the cutting fixture includes two separate slots, one for resecting the medial condyle and the other for resecting the lateral condyle. In all embodiments, however a mechanism ensures that the medial and lateral cuts are at all times co-planar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a portion of 3-foot digital standing x-ray used for measurement;

FIG. 2 is a closer view of the line shown in FIG. 1;

FIG. 3 illustrates one embodiment of a cutting fixture constructed in accordance with the invention;

FIG. 4 shows an embodiment of a cutting block that uses a single slot and a plurality of pin holes; and

FIGS. 5A and 5B present a series of drawings which show how the cutting block may be placed manually on the distal femur.

DETAILED DESCRIPTION OF THE INVENTION

This invention resides in apparatus and methods enabling custom cuts on the distal femur without resorting to expensive computer navigation. The method involves measurement, on plain radiographs prior to surgery, the amount of bone that would be resected on each knee, medially and laterally on the distal femur, using a five degree valgus cut. The amount of bone to be resected with this predicted cut is then compared intraoperatively to what we find when we actually make the cuts. Accuracy may be further improved using a digital x-ray with the patient standing. This x-ray is taken from hip, including the knee and the foot, on one film. A line is then drawn from the center of the femoral head to the center of the distal femur and the intracondylar notch. A perpendicular is taken to this line, with a reference point of 10 mm of resection taken arbitrarily from the medial side. This typically gives a resection somewhere in the 6 to 7 mm range on the lateral side, but this may be different for each patient. From my own personal experience, the “best” knees have 3-4 mm less resected on the lateral distal femur. This measurement could be made and the measurement that would be obtained would then be ‘reverse engineered’ at the time of surgery.

The method uses an inventive cutting block that is pinned on the distal femur and adjusted to perform the resections on both sides. On the medial side, the goal would be 10 mm, more or less. Longer distances in the range of 10-12 mm, or shorter distances in the range of 5-9 may sometimes be used depending upon anatomy. On the lateral side, the depth of the cut is based on the x-ray measurement. This procedure should achieve ideal coronal balance for that patient. This method should yield the same result as a computer navigated knee without having the expense or the time needed to go through all that is needed in terms of setup and equipment.

The cutting block includes a central pin which would be placed on the distal femur. The cutting block is adjusted so that measured resection could be made medial and lateral, and then pinned in place and the final cuts could be made. The anterior cut on the femur may be made using an instrument which sits on the anterior cortex of the femur with a 3-degree upward slope to avoid notching of the anterior cortex of the femur.

The ultimate accuracy of the technique is achieved with a three-foot standing x-ray, preferably a digital x-ray which would take the film from hip, including the knee, down to the ankle, with a line drawn from the center of the femoral head perpendicular measurement made at the distal femur. Measurements obtained on that x-ray could then be used in the operating room to yield a predicted cut.

FIG. 1 depicts a portion of 3-foot digital standing x-ray used for measurement. A point 102 in the center of the femoral head is marked as shown, and a line 104 is drawn from the center of the femoral head to the center of the knee using the tools for measuring the digital films available on most digital software programs. This line, from the center of the hip to the center of the knee, defines the “mechanical axis” of the knee. An “orthogonal measurement tool” is used to draw a line 106 perpendicular to line 104. This line is then placed at the level of the knee to determine how much bone should be resected in accordance with the invention.

FIG. 2 is a closer view of the line shown in FIG. 1. Ten (10) millimeters (mm) is measured from the medial femoral condyle as shown by the line marked “2.” The distance from the lateral femoral condyle is then measured using the calibrated measuring tool on the computer software. The line labeled “3” shows the measurement of the predicted cut of the lateral femoral condyle along the line 106 perpendicular to the mechanical axis, thereby establishing the correct cut for the mechanical axis of this patient's unique anatomy. In this case, the measurement of lines 2 and 3 turns out to be 10 mm and 6.6 mm, respectively. This measurement will predetermine the cuts for the distal femur at the time of surgery to achieve the correct alignment for this patient.

To make these cuts based on the predicted values involves the use of a unique cutting block and system. This system and instrument allows the measured resection to take place medially and laterally. By convention, the medical resection will be either 8 or 10 mm. The lateral resection will be variable, however, and will be determined by the value arrived at by analysis of the digital film lateral femoral condyle cut.

FIG. 3 illustrates one embodiment of a cutting fixture constructed in accordance with the invention. The system includes a medial cutting guide 302 with a first saw-receiving slot 303. The medial guide 302 includes a fixation feature such as a pinhole 310 coupled to the guide through link member 306. The use of a pinhole enables the guide to rotate about point 310. The medial guide 302 further includes a stop element 311 which, when positioned against the most lateral aspect (i.e., apex) of the medial condyle, fixes the cutting slot at a fixed distance such as 8 or 10 mm.

The medial guide 302 is interconnected to a lateral cutting guide 304 having slot 305 through a linkage 314. The linkage 314 enables the lateral guide 304 to be moved toward and away from the medial guide 302, and adjusted in the proximal-to-distal dimension while at all times keeping the cutting slots 303, 305 coplanar to one another.

One portion of the distal guide 304 includes a proximal-to-distal depth gauge 320 which measures the depth of the lateral cut. This guide is adjusted to match the correct depth of cut measured from the using the calibrated measuring tool on the computer software as discussed above. In the disclosed example, the depth would be adjusted to read 6.6 mm, at which point the lateral guide would be pinned in position at 312. A caliper may also be used to measure the amount of bone to be resected from the lateral side. The slots are checked to ensure that they are co-planar, after which both cuts are made.

FIG. 4 shows an embodiment of a cutting block 402 that uses a single slot 404 and a plurality of pin holes. The block 402 is first pinned in an initial hole aided by a stylus 410 which is temporarily placed medially in the slot to show a predetermined distance such as 8-10 mm. As with the embodiment of FIG. 3, the block is rotated about the initial pin hale until the correct distance is shown on the lateral side (i.e., 6.6 mm in the disclosed example). The stylus 410 may be moved over and used for this measurement as well. Once the two distances are correct the cuts are made.

FIGS. 5A and 5B present a series of drawings which show how the cutting block may be placed manually on the distal femur. FIG. 5A is a side view of the apparatus, and FIG. 5B is a top-down view. In this case the flexion/extension of the block will be determined by a ‘foot’ 502 that fits on the anterior cortex 504. This anterior cortex reference provides the flexion/extension of the cut, adding three degrees of flexion to avoid notching of the distal femoral cortex. The guide includes a first arm 506 that attaches to the cutting block portion 508 through an adjustable slot 510 that facilitates proximal-distal excursion as well as rotation to swing varus or valgus. One or more stylus components 520 may be provided with a first end 522 that fits into the slot 512 of the cutting block and a second end that rests against the apex of a given condyle. The second end 524 may vary in thickness to give cuts of, say, 4-12 mm in 1 mm increments. The proximal-distal adjustment allows different thicknesses of cuts to be made based on the thickness of the predetermined sizes.

In use, a stylus component would be placed in the medial slot to resect at a given distance such as 8-10 mm, continuing the disclosed example. The correct distance for the later side (i.e., 6.6 mm in this case) could either be measured or a second stylus component having the correct dimensions could be used. Once the medial and lateral distances have been set in conjunction with varus or valgus adjustment, the cutting block portion 508 is pinned with pin holes 516 and the cuts are made. 

1. A method of resecting a distal femur in conjunction with total knee replacement surgery, comprising the steps of: obtaining an x-ray image of a patient's femur, the image including a hip joint with a femoral head and a knee joint with medial and lateral condyles and an intercondylar notch; identifying a first line on the image from the center of the femoral head to the center of the distal femur and the intercondylar notch; identifying a second line perpendicular to the first line, the second line being at a predetermined, arbitrary distance from the apex of the medial condyle; measuring the resulting distance from the second line to the apex of the lateral condyle; providing a cutting fixture to resect the medial condyle at the predetermined distance and the lateral condyle at the measured resulting distance; and resecting the medial and lateral condyles using the fixture.
 2. The method of claim 1, wherein the predetermined distance is in the range of 8-10 mm.
 3. The method of claim 1, wherein the predetermined distance is approximately 10 mm.
 4. The method of claim 1, wherein the predetermined distance is approximately 10 mm and the resulting distance from the second line to the apex of the lateral condyle is in the range of 6 to 7 mm.
 5. The method of claim 1, wherein the cutting fixture includes: at least one cutting slot; and at least one device for measuring the distance between the apex of the medial condyle and the cutting slot.
 6. The method of claim 1, wherein the cutting fixture includes: at least one cutting slot; and at least one device for measuring the distance between the apex of the medial condyle and the cutting slot and the distance between the apex of the lateral condyle and the cutting slot.
 7. The method of claim 1, wherein the cutting fixture includes separate slots for the resecting the medial and lateral condyles.
 8. The method of claim 1, wherein the cutting fixture includes: two separate slots, one for resecting the medial condyle and the other for resecting the lateral condyle; and one or more devices for measuring the distance between the apex of the medial condyle and the slot for resecting the medial condyle and for measuring the distance between the apex of the lateral condyle and the slot for resecting the lateral condyle.
 9. The method of claim 1, wherein the cutting fixture includes: two separate slots, one for resecting the medial condyle and the other for resecting the lateral condyle; and a mechanism for ensuring that the two slots are at all times co-planar.
 10. The method of claim 1, wherein the cutting fixture is initially rotatable to first adjust for the predetermined distance.
 11. The method of claim 1, wherein the cutting fixture includes an arm configured for placement on the outer cortex of a femur.
 12. The method of claim 1, including the step of obtaining a digital, standing x-ray image of a patient's femur. 